The present invention relates to a method and an apparatus for injection molding of various thin components, such as optical disk substrates and magnetic recording mediums, which are made of synthetic resin and have relatively wide flat surfaces.
In a conventional injection molding machine as shown in FIGS. 7A and 7B, optical disk substrates are formed in a cavity 53 between two molds 51 and 52 at a stationary plate side and at a movable plate side, respectively. In a curing process of a resin material during the molding operation, a sprue portion 54a is separated from the optical disk substrate 54 by lifting up a gate cut punch 62 mounted at the center of the movable plate from a position shown in FIG. 7A to a position shown in FIG. 7B. The two molds are then opened after the optical disk substrate 54 is cooled down and cured by releasing heat to the molds. Generally, in order to minimize the duration required for the molding cycle, a jet of air is applied from slots 55 in the mold to the interface between the optical disk substrate 54 and the mold 51 while the molds are being opened at a high speed so as to swiftly cool down and separate the optical disk substrate 54 from the mold 51 at the stationary plate side.
After the separation of the two molds, the optical disk substrate 54 and the sprue portion 54a are ejected out by an eject rod (not shown), which is mounted at the center of the movable plate, lifting up in a sequence a sprue eject rod 56 and a substrate eject rod 57, both being joined to the mold 52, as shown in FIGS. 8A-8C, while a stream of air is applied from slots 58 in the mold 52 to the other side of the optical disk substrate 54 to ease the removal of the optical disk substrate 54 from the mold 52.
After the completion of the ejecting operation, the optical disk substrate 54 and the sprue portion 54a are transferred from the injection molding machine to the outside by a pickup device (not shown).
There is a demand for further reduction of the molding cycle. In order to achieve this object, it is essential to minimize the time for cooling. The cooling time at the time of separating the molds is, however, substantially short, and further reduction of the time may lead to undercooling of the substrates. If the temperature during the separation of the molds and the ejection of the optical disk substrate 54 becomes higher than a thermoforming temperature of the resin material, the resistance generated when the optical disk substrate 54 is separated from the molds 51 and 52 or the moment generated between the center and the rim of the optical disk substrate 54 when its central part is lifted up while the rim is not fully separated from the mold may cause a deflection or bend in the optical disk substrate 54 as shown in FIG. 8C.